1. The Field of the Invention
The present invention relates generally to medical devices and methods of bone growth stimulation. More particularly, the present invention relates to devices, systems, and methods for non-invasive bone growth stimulation using a constant-current capacitively-coupled stimulator.
2. The Relevant Technology
The two most prominent methods of stimulating bone growth across failed unions of fractures of bones are to subject them to an ultrasonic stimulus or an electrical stimulus. Electrical stimulus seems to be the preferred therapy for long bone fractures that have failed to heal. Small and flat bones seem to respond better (or more conveniently) to ultrasonic therapy.
The electrical stimulus of long bone fractures that have failed to respond and heal with conventional therapies, such as fixation, pinning, bone grafts, etc. are divided between two primary methods. The oldest method is pulsed electromagnetic stimulation (PEMS). PEMS requires the area of the fracture to be encased in a fixture containing an electrical coil through which a series of pulsed electrical currents in a rather sharply defined wave pattern are passed. There are many clinical studies that validate the effectiveness of this method of treatment. PEMS usually requires that the patient remain non-ambulatory during each treatment session. The equipment is not very portable and must be carefully size adjusted to each patient application.
The second method of providing electrical bone growth stimulus to long bone fractures is through capacitance coupled electrostatic stimulation (CES). CES treatment requires the placement of two electrodes, typically approximately 30 mm in diameter, in axial alignment with the fracture and in radial opposition to each other across the fracture site. An AC electrical signal is applied to the electrodes.
The CES method of treatment is far more flexible in application than the PEMS method and allows the patient, if otherwise ambulatory, to go about life as usual while undergoing treatment from a portable device that can in many cases weigh less than one pound.
Conventional CES devices use a constant voltage approach. That is, the signal provided to the electrodes is generated such that the voltage remains substantially constant over time, typically at 5 volts, peak-to-peak. In such devices, the current typically fluctuates so as to keep the voltage constant. It appears, through clinical trials, that the most effective electrical signal in a CES system in promoting the healing of a failed long bone fracture appears to be a 60.0 kHz sine wave that delivers between 5.0 and 7.5 milliamps, RMS, to the electrodes. It appears that delivered currents less than 5.0 milliamps, RMS, fail to produce the desired results. Some clinical indications have further shown that currents in excess of 10.0 milliamps, RMS, may cause harm and even some bone death. Thus, to produce a therapeutic benefit, the CES current should fall within the narrow range of between 5.0 milliamps RMS and 10.0 milliamps RMS. As such, there is a need in the art for a CES device that can produce a current to the electrodes that is substantially constant and doesn't vary below 5.0 milliamps or above 10.0 milliamps, RMS.